Rack Leaders/Framing/VFX Charts

This is a bit off-topic from steadicam-related things, and more of a camera assistant thing.

With the prevalence of of VFX heavy shows, assistants are often being asked to shoot VFX charts and grids. Often times they involve shooting it with all the lenses in the package and depending on each focal length, you'll need to move the camera to an appropriate distance to frame the chart.

My question is how do you keep the focal plane of the camera centered and completely parallel with the chart?

This problem also crops up when you are required to shoot a rack leader/framing chart.

I can mount the chart on a wall. But there is no guarantee the wall is absolutely perpendicular to the floor. The floor you set up the camera on may not be perfectly flat.

I can use a level and level the camera and the chart, measure each of their center heights. But how do you ensure you are at the center of the chart left-to-right (even at distances of 20 or 30 feet), and ensure the pan/tilt of the camera isn't off-axis to the chart?

With film cameras, you could shine a light down the viewfinder and project the ground-glass on the wall. But you can't do that with digital cameras.

So, I'm curious how other assistants have done it. What is the method to maintain relatively high accuracy while being practical and not taking 20 hours to shoot VFX charts?

I have done this before with a small shaving mirror mounted flat to the chart in the center. A lens light on the camera can help find where the camera roughly is (it can be hard to find at the really long distances), but essentially find the front element of the lens in the reflection in the tiny mirror. So long as the mirror is mounted flush to the chart and the chart is perfectly flat, there should be nothing else you need to do to get perfectly square at any distance. The only thing this doesn't help with is the 'roll' axis, but I find levelling the chart and camera should take care of this axis.